Adhesives have been used for a variety of marking, holding, protecting, sealing and masking purposes. Adhesive tapes generally comprise a backing, or substrate, and an adhesive. One type of adhesive, a pressure sensitive adhesive, is particularly preferred for many applications. Pressure sensitive adhesives (PSA's) are well known to one of ordinary skill in the art to possess certain properties at room temperature including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength.
Materials that have been found to function well as pressure sensitive adhesives are polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear strength. The most commonly used polymers for preparation of pressure sensitive adhesives are natural rubber, synthetic rubbers (e.g., styrene/butadiene copolymers (SBR) and styrene/isoprene/styrene (SIS) block copolymers), various (meth)acrylate (e.g., acrylate and methacrylate) copolymers and silicones.
General purpose tapes which stick to all types of surfaces and especially pressure sensitive adhesives which stick very well to Low Surface Energy substrates typically require addition of high amounts of tackifying resins. PSA's prepared from solution polymer may compensate for the reduced cohesive strength, due to the presence of low molecular weight tackifying resin, with appropriate addition of crosslinkers or increased molecular weight of the polymer. In hot melt processable formulations though, the polymer has to be able to flow sufficiently at extruder temperature and therefore the maximum molecular weight is limited. Furthermore, the combination with thermal crosslinkers to create a higher cohesive strength via an increase of the molecular weight and the creation of a chemical network is not always practical because of its potential implications during hotmelt processing.
It is known that crosslinking of polymers produces polymer networks which have quite different mechanical and physical properties compared to their uncrosslinked linear or branched counterparts. For example, polymer networks can show such unique and highly desirable properties as solvent resistance, high cohesive strength, and elastomeric character.
Crosslinked polymers can be made in situ during formation of the desired polymer product. Many patents are known describing techniques to achieve efficient crosslink mechanisms and good cohesive strength properties. Therefore, the problems associated with solvent and bulk processing of crosslinked materials may be avoided through the use of actinic radiation processing. U.S. Pat. No. 4,379,201 (Heilmann et al.) discloses an example of a class of polyacrylic-functional crosslinkers used in the photocuring of (meth)acrylate copolymers.
U.S. Pat. No. 4,391,678 (Vesley) and U. S. Pat. No. 4,330,590 (Vesley) describe a class of fast curing triazine photocrosslinkers which, when mixed with an acrylic monomer and, optionally, a monoethylenically unsaturated monomer, and exposed to UV radiation, forms a crosslinked polyacrylate. The crosslinks formed by both the (meth)acrylates and the triazines in these copolymerizations prevent any further processing, such as hot melt coating, reactive extrusion, or solution coating processes, following the initial photopolymerization. However, since further processing of the polymer product is often necessary, it is more typical to start from the linear or branched polymer which, in the final processing step, is cured to a crosslinked material. The curing or crosslinking step is typically activated by moisture, thermal energy or actinic radiation. The latter has found widespread applications, particularly in the use of ultraviolet light as the radiation source.
In the past, a variety of different materials have been used as crosslinking agents using actinic radiation, e.g. polyfunctional acrylates, acetophenones, benzophenones, and triazines. The foregoing crosslinking agents may however possess certain drawbacks which include one or more of the following: high volatility; incompatibility with certain polymer systems; generation of corrosive or toxic by-products; generation of undesirable color; requirement of a separate photoactive compound to initiate the crosslinking reaction and high sensitivity to oxygen.
In addition to actinic radiation processing described above, acrylate PSAs can be applied to substrates by solvent and hot-melt coating techniques. Although solvent coating techniques are widely used, hot-melt coating techniques may provide some environmental and economical advantages. However, unlike solvent coating techniques where the polymer coating and crosslinking are performed simultaneously, hot-melt coating requires that coating and crosslinking be performed sequentially. This is due to competing considerations: a polymer should not be highly crosslinked if it is to be hot-melt coated smoothly, yet the polymer needs to be crosslinked to achieve certain desirable performance properties such as e.g. high shear when the polymer is a PSA. Therefore, hot-melt coating is performed prior to crosslinking of the coated polymer.
Because hot-melt coating techniques involve high amounts of thermal energy and shear, the subsequent crosslinking procedure usually involves non-thermal energy sources. Electron beam (e-beam) and ultraviolet (UV) energy sources have been used traditionally, although e-beam techniques often are too energy intensive to be practical. Accordingly, much interest has been focused on UV radiation crosslinking of polymers.
UV radiation crosslinking of coated polymers has relied almost exclusively on hydrogen abstraction techniques in which a hydrogen abstracting agent, such as e.g. benzophenone or anthraquinone, is blended into the coated mixture prior to or during the coating process, and the mixture is then exposed to appropriate UV radiation. Certain polyfunctional benzophenones have been investigated as photocrosslinking agents and/or photosensitizers in various photopolymerizable systems.
To date, there is no disclosure of any highly tackified radiation crosslinkable PSA formulations, in particular solventless PSA formulations, which include an incorporated photocrosslinker at levels higher than 0.05 parts by weight per 100 parts of polymeric material, and which after suitable radiation crosslinking provides a highly tackified radiation crosslinked pressure sensitive adhesive in particular provides high cohesive strength at elevated temperature and high-temperature shear resistance whilst ensuring excellent adhesion to various types of substrates.
Also, when a tackifying resin is present in the PSA formulation, especially in a relatively high amount, a large fraction of the exposed UV light during the crosslinking step is absorbed by the tackifying resin/photocrosslinker system which may result in reduced crosslinking efficiency and poor cohesive strength of the resulting PSA. When radiation crosslinking is used to crosslink tackified PSA formulations, the tackifying resin may provoke several other deleterious effects such as e.g. undesired chain transfer or chain termination reactions. The use of high levels of tackifying agent(s) may be desirable because it can increase the tackiness of the pressure sensitive adhesive, making it aggressively adhere to wide range of substrates without the need to apply pressure. The addition of tackifying resin, especially high levels of tackifying resin, may detrimentally affect the shear and cohesive strength of a pressure sensitive adhesive, and may even raise the Tg of the adhesive. The use of high levels of tackifying resin may be particularly detrimental to hot melt processable pressure sensitive adhesives where the need to be hot melt processable can already adversely affect the shear strength and cohesive strength properties of the adhesive.
U.S. Pat. No. 4,737,559 (Kellen et al.) discloses a PSA which is a copolymer of an acrylate monomer and a copolymerizable mono-ethylenically unsaturated aromatic ketone comonomer free of ortho-aromatic hydroxyl groups. WO-A1-97/40090 (Stark et al.) describes a radiation crosslinkable composition comprising: a) a radiation crosslinkable polymer having abstractable hydrogen atoms and radiation-activatable crosslinking groups capable of abstracting hydrogen atoms when activated; and b) a non-polymerizable radiation-activatable crosslinking agent capable of abstracting hydrogen atoms when activated. WO-A1-96/35725 (Carpenter) discloses pigmented, UV-crosslinked, acrylic-based, pressure sensitive adhesives claimed to have high cohesive strength and high-temperature shear resistance. The adhesives disclosed in WO-A1-96/35725 comprise an acrylic copolymer compounded with a pigment and a hydrogen-abstracting photoinitiator, wherein the acrylic copolymer is obtained by copolymerizing an alkyl acrylate and a tertiary amine-containing monomer. WO-A1-2012/044529 (Satrijo et al.) describes a hot-melt processable PSA comprising: a) a hot-melt processable elastomeric (meth)acrylate random co-polymer; b) at least one tackifing resin comprising greater than 50 parts by weight per 100 parts by weight of elastomeric (meth)acrylate random co-polymer; and c) a thermoplastic material.
Without contesting the technical advantages associated with the solutions disclosed in the art, there is still a need for a highly tackified radiation crosslinked pressure sensitive adhesive which overcomes the deficiencies previously mentioned, and which in particular provides high cohesive strength at elevated temperature and high-temperature shear resistance whilst ensuring excellent adhesion to various types of substrates.
Other advantages of the pressure sensitive adhesives, precursors of pressure sensitive adhesive and methods of the invention will be apparent from the following description.